Magnetic discs with magnetizable media are used for data storage in most computer systems. Current magnetic hard disc drives operate with the read-write heads only a few nanometers above the disc surface and at rather high speeds, typically a few meters per second.
Generally, the discs are mounted on a spindle that is turned by a spindle motor to pass the surfaces of the discs under the read/write heads. Spindle motors generally include a shaft, a base plate, a sleeve, and a hub. The shaft may be separate from and attached to the hub or incorporated in the hub. The shaft and the sleeve rotate relative to each other. Either the shaft rotates and the sleeve remains stationary, or vice versa. Permanent magnets attached to the hub interact with a stator winding on the base plate to rotate the shaft or the sleeve. To facilitate relative rotation of the shaft and the sleeve, one or more bearings are usually disposed between the shaft and the sleeve.
FIG. 1 shows a schematic of a magnetic disc drive with a spindle motor that commonly utilizes a fluid dynamic bearing. Fluid bearings use a thin layer of liquid or gas fluid between the bearing faces. Referring to FIG. 1, a disc drive typically includes a housing having a base sealed to a cover with a seal. The disc drive has a spindle to which are attached one or more discs having surfaces covered with a magnetic media (not shown) for magnetically storing information. A spindle motor (not shown in this figure) rotates the discs past read/write heads, which are suspended above surfaces of the discs by a suspension arm assembly. In operation, the spindle motor rotates the discs at high speed past the read/write heads while the suspension arm assembly moves and positions the read/write heads over one of a several radially spaced tracks (not shown). This allows the read/write heads to read and write magnetically encoded information to the magnetic media from and to the surfaces of the discs at selected locations.
As illustrated in the spindle motor embodiment of FIG. 2, spindle motors can include a shaft having an outer surface that abuts a sleeve. The shaft rotates relative to the sleeve or vice versa. In this embodiment, the shaft is separate from the hub; however, the hub and shaft may have a one-piece construction.
Fluid dynamic bearings are commonly located between the hub/shaft (which refers to a one-piece hub-shaft embodiment and an embodiment with separate hub and shaft) and the sleeve of the motor, which typically move relative to each other. The fluid used in the bearing is commonly an oil, and fluid type choices are usually based on the fluid's viscosity, as well as its equilibrium vapor pressure and the coefficient of gas phase diffusion. The fluid is also referred to herein as lubricant. Fluid dynamic bearing designs are known to use a capillary seal to contain a volume of lubricant necessary for continuous and proper motor operation. FIG. 3 illustrates a vertical cross section of a prior art spindle motor having a fluid dynamic bearing with a capillary seal. This spindle motor embodiment includes a one-piece hub and shaft. Typical bearings lubricants are optimized to have lower viscosity, which decreases motor power consumption and enhances motor performance. In addition, the lubricant preferably has a low equilibrium vapor pressure to reduce fluid loss by evaporation. However, lower viscosity lubricating fluids typically have higher vapor pressure that results in a substantial amount of lubricant being lost from the capillary seal by evaporation and oil vapor diffusion in the gas phase over the life of the motor.
To compensate for lubricant loss, capillary seal are often designed to hold a larger amount of lubricating fluid. The available reservoir volume is, however, limited by motor size constraints and requirements for seal splash robustness during shock events. It is advantageous to minimize the amount of fluid that evaporates from the capillary seal over the life of the motor. This is preferably done without negatively affecting spindle motor performance afforded by low viscosity fluids.